In June 2005 the General Lighthouse Authorities (GLAs) for Great Britain and Ireland (Trinity House, the Northern Lighthouse Board and the Commissioners of Irish Lights) commenced trials of Loran-C near Rugby in the English Midlands.

ALONG with other bodies, the GLAs remain concerned by the sole reliance of mariners on satellite systems, and the vulnerability of transmissions from satellites. The trial Loran-C transmitter will enable the GLAs to test the efficacy of Loran-C in the Irish and British areas of operation and evaluate it as a complementary system to the Global Navigation Satellite System (GNSS). Loran-C is dissimilar to GNSS in many respects, relying on low frequency transmissions from transmitters on the ground. Because of its different characteristics it can provide an effective back-up and be complementary to GNSS.
Loran-C uses low frequency terrestrial transmissions and provides another source of navigation information should satellite navigation systems be unavailable. The trial will run until March 2007 and the transmissions will be integrated with those of other transmitters in Europe.

There is increasing awareness that, because of the very low signal levels that reach receivers, GNSS is vulnerable to both intentional and unintentional interference and this, therefore, renders total dependence on GNSS unwise.
GNSS systems also suffer occasionally from undetected failures; for example, a satellite clock error on 1 January 2004 gave rise to errors in measured positions of up to 45km in western Europe for a period of some three hours, resulting in onboard navigational failures.

Loran-C can provide position information entirely independent of GNSS. The two systems' positions can be compared and an alarm generated if they diverge beyond a set limit. Several equipment manufacturers supply combined GNSS/Loran-C receivers which produce position information from either system, or both. Commonly, GNSS is used as the primary system, with Loran-C providing an automatic back-up.

Each Loran-C transmitter station has its own standard frequency clock, independent of GNSS. The Loran-C signal is transmitted at much higher power and a lower frequency than GNSS, making it much less vulnerable to interference.
Loran-C can produce position information in the same format as GNSS, so the two provide alternative inputs to electronic charts, Global Maritime Distress and Safey System radios and other onboard systems.
The draft European Radio Navigation Plan recommends that the eu should work with member states to investigate the Europe-wide provision of Loran-C services in order to secure the transport and wider socio-economic policy benefits that Loran-C promises.

Loran-C currently provides a nominal minimal accuracy of some 400m within the coverage area. In practice its accuracy, which depends on the user's position relative to the transmitter sites, is generally considerably better than this. Recent tests suggest that in future this accuracy will be further improved using a combination of spatial and temporal corrections, in developments known as enhanced Loran, or eLoran. The accuracy attainable with eLoran is likely to be of the order of 20m.

Material originally published in the 2005/3 edition of IALA Bulletin is reproduced by kind permission of the Editor.